I have always heard that if you stay on an antibiotic for a long period of time, your body becomes immune to it.....so when you really have a bad infection in the future, the antibiotic might not work.

This is true, however its the specific bacteria that mutate and become immune. So generally, if this becomes immune, you try another antibiotic. Another, unrelated infectious bacteria, would not be mutated. There are thousands of different specific antibiotics, new ones being discovered/made all the time. It is scary how fast and how many antibiotics have become useless due to mutation.